1. Field of the Invention
The present invention relates to a variable-modulus material suitable for automotive parts such as strut mount, engine mount, and shock absorber as vibration dampers. It will also find use as vibration insulators for electronic balances, laser scanning microscopes, scanning tunneling microscopes, and fuel and circulating pumps. It is capable of controlling predictable or unpredictable vibrations and shocks because it changes in modulus instantaneously or in a fixed period of time.
2. Description of the Related Art
By "variable-modulus material" as used herein is meant a functional polymeric material which rapidly changes in modulus upon application of an external electric field. It undergoes a reversible modulus change as the result of conversion of electrical energy applied thereto into mechanical energy. Therefore, it absorbs externally applied vibrating energy and hence functions as a vibration damper.
Automobiles are subject to low-frequency large-amplitude vibrations (due to road bumps) and high-frequency small-amplitude vibrations (during a high-speed running). Controlling or suppressing such vibrations is necessary to improve ride comfort and maneuverability. To this end, there has been proposed a vibration-proof bushing made of a material which changes in vibration-absorbing performance according to the running conditions of the vehicle. (Japanese Patent Laid-open No. 107334/1992) There has also been proposed a material which varies in vibration-absorbing capacity upon application of an electric field (Japanese Patent Laid-Open No. 91541/1991 corresponding to U.S. application Ser. No. 26,211) or upon application of a magnetic field (Japanese Patent Laid-Open No. 25316/1993). These variable-modulus materials are polymeric materials of particle-dispersed type. They increase in modulus upon application of an electric or magnetic field which brings about the polarization of dispersed particles and the bonding of dispersed particles due to the dipole-dipole interactions between dispersed particles. There is known a suspension (electrorheological fluid) which becomes more viscous when exposed to an electric field. (Japanese Patent Laid-open Nos. 216202/1986 and 139639/1989). The electrorheological fluid also increases in apparent modulus due to the bond strength between dispersed particles which is produced by the external electric field. Therefore, it will also find use as strut mounts, engine mounts, and shock absorbers, as well as the above-mentioned automotive vibration-proof bushings.
The above-mentioned variable-modulus material of dispersed-particle type or electrorheological fluid needs a high voltage of 1-5 kV/mm for its modulus change by electric field. Such a high voltage cannot be generated by an automotive battery, whose normal voltage is 12 V or 24 V, without the help of an expensive high-voltage generator. The magnetic field-induced variable-modulus material also needs an electromagnet for its modulus change by magnetic field. Such an additional generator or electromagnet augments the production cost and complicates the structure.
The above-mentioned variable-modulus material increases in modulus (or becomes stiffer) when exposed to an electromagnetic field; however, there is an instance where it is desirable that the variable-modulus material as a vibration insulator becomes soft occasionally as required.